On 05/21/2012 09:47 AM, Paolo Bonzini wrote:
Il 21/05/2012 16:40, Anthony Liguori ha scritto:
On 05/21/2012 09:26 AM, Paolo Bonzini wrote:
Il 21/05/2012 16:19, Anthony Liguori ha scritto:
I'm not against it in principle, just in practice. Today, checking
whether a command exists is:
commands = qmp.query_commands()
if 'block-stream' in commands:
# has block-stream
I have a hard time envisioning how schema introspection can be
reasonably implemented in a client.
schema = qmp.query_command_schema('block-stream')
What would schema return?
Did you mean:
if schema['arguments'].has_key('on_error'):
Yes, something like that.
What about adding a parameter to a structure?
schema = qmp.query_type('foo')
if schema['data'].has_key('xyz')
This may end up being workable. I hadn't thought of having granular QMP schema
querying functions.
BTW, the other problem with adding arguments like this is that it makes
a stable C API impossible.
Adding fields to structs is not a problem as long as "libqmp" takes care
of all allocations on part of its client.
Yes, the previous version I wrote handled this padding structures appropriately.
Adding parameters to commands
requires some smartness, but there are ways to do it:
1) add the first version number to the schema, generate versioned entry
points
qmp_block_stream_v1_1
qmp_block_stream_v1_2
etc. Provide multiple headers libqmp-1.1.h, libqmp-1.2.h etc. that take
care of #define'ing qmp_block_stream to one of them
This is pretty nasty :-/ It also gets very challenging if some options are
backported and others aren't.
I guess let me ask the following question:
Is the problem that should have an 'options' parameter to this command? Do we
anticipate future addition of arguments? If we can at least minimize the C API
breakages, I'd be happy.
Regards,
Anthony Liguori
2) Same as (1) but use qmp_block_stream for the oldest version having
the command.
3) have fun with the preprocessor (macro with variable arguments,
sizeof, designated initializers, whatever) and emulate keyword arguments
for the C API.
Paolo
